The present invention relates generally to the field of measuring distance between two communicating stations and, more particularly, to the field of measuring distances between a moving aircraft and a stationary ground station.
There is currently provided an internationally accepted distance measuring system for use in an L-band frequency spectrum which can provide some measurement of distance between two communicating stations. However, the utility of this system is greatly limited because its distance measurement accuracy is inherently too poor to satisfy the evolving needs of military and civil aviation.
Additionally, some users of distance measuring equipment (DME) do not require greater accuracy than currently provided by the conventional system and wish to avoid the costs of any additional DME needed to provide greater accuracy. Most users do desire some degree of improvement in accuracy and indeed some users require accuracies which are well beyond the capability of any modifications which can be made to the conventional system while preserving its compliance with L-band frequency spectrum requirements.
Proposals for improving the distance measuring ability of the conventional system have included a "precision" L-band DME currently being tested as part of the Federal Aviation Administration National Microwave Landing System Program. However, this improved system is plagued by a relatively high cost. Another proposed system is described in Federal Aviation Association Document FAA-ER-700-08 as a "chirped" C-band DME for use with a microwave landing system (MLS). Again, users not requiring high accuracies have objected to the higher cost which they believe will be incurred from the utilization of techniques capable of achieving a high accuracy.
Further problems incurred with several of the proposed systems, in addition to their cost, are their power requirements and weight, which are both significant in aircraft installations.